1. Field of the Invention
This invention relates to a strained quantum well type semiconductor laser device to be used in the field of opto-electronics including optical telecommunication and data telecommunication.
2. Prior Art
Strained quantum well type semiconductor laser device comprising an active layer having a strained quantum well structure are categorized as compound semiconductor laser devices and massive studies are being currently carried out on them.
As is well known, a quantum well type semiconductor laser device comprises an active layer having a thickness as small as the wavelength of Broglie waves and a quantized level for electrons to take as a function of the well type potential of its width.
Such an active layer is required to have a thickness of approximately 100.ANG. in order to show an excellent performance. Such a thin active layer can be prepared by means of a crystal growth method such as MOCVD or MBE.
In a strained quantum well type semiconductor laser device, the lattice constant of a quantum well layer and that of an adjacent barrier layer of its strained quantum well structure normally differ from each other by approximately 0.5 to 2%.
In a strained quantum well type semiconductor laser device as mentioned above, the in-plane hall effective mass in the valence band comes close to that of an electron in the conduction band and consequently the laser device can operate with a lower threshold current.
It is a proven fact that a strained quantum well type semiconductor laser device has a low threshold current for an oscillation wavelength of approximately 1 .mu.m.
In view of this fact, there has been a strong demand for strained quantum well type semiconductor laser devices that operate similarly well at the lower threshold current for oscillation wavelength bands of not only 1.3 .mu.m and 1.5 .mu.m which are particularly important for optical telecommunication, but also 0.63 .mu.m for optical disc application.
For the oscillation wavelength band of 1.5 .mu.m, a strained quantum well type semiconductor laser device comprising quantum well layers made of compressively strained InGaAs and barrier layers made of strain-compensated InGaAsP has been proposed.
FIG. 3 of the accompanying drawings illustrates such a strained quantum well type semiconductor laser device which is generally denoted by reference number 12 and comprises an n-InP buffer layer 14, a nondoped InGaAsP optical waveguide layer 15, a quantum well active layer 16, a nondoped InGaAsP optical waveguide layer 17, a p-InP clad layer and a p-InGaAsP contact layer 19 are sequentially formed in the above mentioned order on an n-InP semiconductor substrate 13 by a crystal growth method (including an epitaxial growth method) such as MOCVD or MBE.
Referring to FIG. 4, the quantum well active layer 16 comprises a number of compressively strained InGaAs quantum well layers 20 and strain compensated InGaAsP barrier layers 21 stacked alternately to produce a multilayered quantum well structure.
Each of the optical waveguide layers 15 and 17 have an energy band gap wavelength of 1.3 .mu.m.
The amount of strain in the well 20 is 1.8% in compression corresponding to x=0.8.
If the thickness of each of the compressively strained layers (InGaAs quantum well layers 20) exceeds 20.ANG., it becomes impossible to obtain .lambda.=1.5 .mu.m.
For an optical semiconductor device comprising compression strained InGaAs quantum well layers 20 and having .lambda.=1.5 .mu.m, each of the quantum well layers 20 cannot have a thickness thicker than 20.ANG. under the condition of x=0.8.
The quantum well layers 20 of a strained quantum well type semiconductor laser device 12 prepared under such restrictive conditions by turn inevitably have to have a high threshold carrier density due to the thin well thickness.
While a strained quantum well type semiconductor laser device 12 having a configuration as described above may be made to operate highly efficiently with a lower threshold current at room temperature, it may not satisfy the requirements of operation of a lower threshold current, a high efficiency and a high speed modulation at higher temperature because the Auger recombination and intervalence electron absorption of such a device that determines its performance at various temperature levels largely depends on the carrier density of the quantum well layers 20.
Additionally, since each thickness of the quantum well layers 20 needs to be controlled to be 20.ANG., the process of producing such layers requires a high degree of process control.
In view of the technological problems as described above, it is, therefore, an object of the present invention to provide a strained quantum well type semiconductor laser device that can be manufactured without any difficulties and operates satisfactorily at high temperature.